1. Field of the Invention
The present invention relates generally to the construction and use of catheters for interventional and diagnostic procedures. In particular, the present invention relates to methods and apparatus for advancing catheters through restrictions and occlusions within body lumens and cavities.
Atherosclerosis is a form of arteriosclerosis which is characterized by irregularly distributed deposits on the walls of a patient's arteries. Such deposits frequently fibrose and calcify over time, seriously compromising the patient's health.
A number of catheter-based approaches have been developed for diagnosing and treating atherosclerosis and other forms of arteriosclerosis. The most common interventional technique for treating atherosclerosis is balloon angioplasty where a balloon-tipped catheter is introduced to the vascular system, and the balloon expanded within a region of stenosis. Other interventional techniques include atherectomy, where, for example, a catheter having a cup-shaped rotating cutter is introduced to the vascular system and used to severe and capture at least a portion of the stenotic material. Other interventional techniques include laser ablation, mechanical abrasion, chemical dissolution, and the like. Catheter-based diagnostic techniques include ultrasonic imaging where an ultrasonic transducer disposed at the distal end of a vascular catheter is introduced to the region of stenosis.
With most of these techniques, it is necessary to advance the distal end of the catheter at least partly through the stenosed region before the interventional or diagnostic procedure can be commenced. While such initial advancement is often not a problem, it can be very problematic when the occlusion is severe and little or no lumen remains to receive the catheter. Under such circumstances, it is necessary to at least partly recanalize the occlusion before the catheter procedure can begin.
A number of methods for recanalizing severe occlusions have been proposed, including the use of hot-tipped catheters, laser catheters, and drill-tipped catheters. In general, these approaches rely on very aggressive treatment of the stenotic material to open up a passage, where such aggressive techniques can expose the blood vessel wall to significant injury, for example, vessel perforation. The risk of injury is exacerbated by the unconstrained path which the catheter can follow.
An improved technique for advancing an angioplasty catheter into and optionally through a severe occlusions is described in U.S. Pat. No. 4,998,933 (the entire disclosure of which is hereby incorporated herein by reference), which has common inventorship with the present application. A first electrode is disposed at or near the distal tip of the angioplasty catheter and a second electrode is provided on an electrically conductive guidewire. After the guidewire is at least partly advanced into a stenotic material, a high frequency voltage can be applied between the guidewire electrode and the catheter tip electrode in order to generate heat within the stenotic material lying between said electrodes. As the stenotic material is heated, it is softened, thereby allowing easier advancement of the angioplasty catheter.
Although a substantial improvement in the art, the catheter described in U.S. Pat. No. 4,998,933 can cause unwanted shorting of electrical energy by the blood and blood vessel wall during the application of the high frequency voltage. The catheter employs a single discrete electrode at its distal tip. So long as the tip electrode fully contacts the stenotic material, the induced heat will be substantially limited to the stenotic material. If a portion (or all) of the electrode is exposed to the blood vessel wall and/or blood, however, current will begin to flow through the blood vessel tissue and/or blood, causing the undesired shorting of electrical current. Moreover, since both the blood vessel wall and the blood have higher electrical conductivities than the stenotic material, they will carry the current in preference to the stenotic material.
For these reasons, it would be desirable to provide improved apparatus and methods for advancing vascular catheters past severe occlusions in blood vessels and other body lumens. In particular, it would be desirable to provide improved catheters of the type described in U.S. Pat. No. 4,998,933, where the catheter more selectively heats the atheromatous material. It would be further desirable if such catheters were able to discriminate between the atheromatous mass and the blood vessel wall (preferentially heating and ablating the former) so that the catheter would selectively pass through the atheroma as the catheter is advanced through the lumen of the blood vessel. The catheters and methods of the present invention should be compatible with a wide variety of interventional and diagnostic devices, particularly being compatible with angioplasty catheters.
2. Description of the Background Art
U.S. Pat. No. 4,998,933, is described above. European Patent Publication 182,689 and U.S. Pat. No. 4,754,752 describe angioplasty balloon catheters having means for internally heating the balloons. A "hot tip" catheter having a metal tip heated by a laser is described in Cumberland et al. (1986) Lancet i: 1457-1459. U.S. Pat. No. 4,654,024, describes a catheter having an electrically heated tip for melting atheroma. U.S. Pat. No. 4,796,622, describes a catheter having a tip which is heated by an exothermic reaction. A catheter having a high speed rotating abrasive element at its distal tip is described in U.S. Pat. No. 4,857,046. U.S. Pat. No. 4,709,698, describes the placement of electrode pairs on the surface of a dilatation balloon to heat atheroma as the balloon is expanded.